Since I was nine on the one occasion I saw this film ( it is aimed at children), I wasn't very aware of such issues as acting and characterisation, so please forgive the rather autistic tone of this review and my memory of it might be poor, but it has often come back to mind over the intervening years. It is an example of a film which is proper "hard" science fiction, which is very rare in cinema or television.
The plot of a science fiction story must depend on its setting, which should be scientifically plausible. If the story could be transferred to any other setting, it is not SF. By this criterion, 'Star Wars' is not SF because the setting could be fantasy or mediaeval without changing the plot, and 'Outlander' is basically a Western in space. 'Into Infinity' could not take place anywhere other than on a spacecraft travelling near the speed of light.
It is also 'hard' SF with a firm scientific basis. Special relativity, the Doppler effect and the evolution of stars are among the real phenomena portrayed in the film. Little is changed from established theories to "improve" the story. Unlike TV series such as Star Trek, there is no technobabble, no unfeasible physics and no made up elementary particles. It is nearly all straight science. There are only a couple of flaws I remember.
Besides these aspects of the story, the look is also quite impressive. I still occasionally dream about some of the scenes, many of which are visually stunning for the time. The sets are futuristic in a very 'seventies style which I will always think of as how the "future" ought to look.
I can't comment on the dramatic merits of the film, though surely you can't go wrong with Brian Blessed, but on the SF side of things I just wish there were more examples of this sort of thing in cinemas and on TV.